Modified aminoplast crosslinkers and powder coating compositions containing such crosslinkers

ABSTRACT

A crosslinking agent is provided, prepared by reacting (A) an aminoplast resin; (B) a reactive urethane group-containing adduct of a polyfunctional polymer and monoisocyanate, and (C) at least one other compound having active hydrogen groups reactive with aminoplast resin (A). Compound (C) is selected from:  
     (i) compounds having the structure (I):  
                 
 
      wherein X is aromatic; R 1 , R 2 , and R 3  each independently represents H, (cyclo)alkyl having from 1 to 12 carbon atoms, aryl, alkaryl, aralkyl, or an active hydrogen-containing group,  
     (ii) compounds having the structure (II) or (III):  
                 
 
      where R′ and R″ each independently represents an aromatic group or an alkyl group having 1 to 12 carbon atoms; and  
     (iii) compounds different from (i) and (ii), having a melting point of at least 80° C.  
     Further provided are methods for preparing the crosslinking agent, curable compositions containing the crosslinking agent, and coated substrates.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Continuation-in-part of U.S. patentapplication Ser. No. 09/666,265, filed Sep. 21, 2000. Reference is madeto related U.S. patent application Ser. Nos. ______; ______; ______;______; and ______, filed concurrently herewith.

FIELD OF THE INVENTION

[0002] The present invention relates to crosslinking agents preparedfrom a modified aminoplast resin and to powder coating compositionscontaining such crosslinking agents.

BACKGROUND OF THE INVENTION

[0003] In recent years, powder coatings have become increasingly popularbecause these coatings are inherently low in volatile organic content(“VOC”), which significantly reduces emissions of volatile organiccompounds into the atmosphere during application and curing processes.

[0004] Hydroxyl, carboxyl, carbamate and/or epoxy functional resins,such as acrylic and polyester resins having relatively high glasstransition temperatures (“T_(g)”), are commonly used as mainfilm-forming polymers for these coatings. Because acrylic polymersystems can be more heat-resistant than condensation polymers, they canprovide powder coating compositions having improved storage stability.However when exposed to the extreme temperatures which can beencountered during shipping and/or storage in many geographic areas,even better powder coating stability is desired. By “storage stability”is meant the ability of the individual powder particles which comprisethe powder coating to resist the tendency to adhere to one another,thereby causing “clumping” or “fusing” of the powder coating compositionupon storage prior to application. Powder coating compositions havingvery poor storage stability can be difficult, if not impossible, toapply.

[0005] Aminoplast resins are well known in the art as low costcrosslinking agents for hydroxyl, carboxyl and/or carbamate functionalpolymers in conventional liquid coating compositions. Common aminoplastresins are based on condensation products of formaldehyde with an amino-or amido-group carrying substance. Examples of these aminoplast resinsinclude the methylol and alkoxymethyl derivatives of ureas, melaminesand benzoguanamines which are most commonly used in liquid coatingcompositions. Such aminoplast resins provide enhanced coating propertiessuch as exterior durability, chemical resistance and mar resistance.

[0006] Attempts to produce powder coating compositions based onconventional aminoplast resins which exhibit these desirable propertieshave been largely unsatisfactory because these materials are typicallyin liquid form and, as such, cause poor powder stability.

[0007] The methoxylated aldehyde condensates of glycoluril, which aresolid products, are the aminoplast resins most commonly employed ascrosslinking agents in powder coating compositions. Although solid inform, these materials nonetheless can depress the T_(g) of the powdercoating composition significantly, even when combined with high T_(g)film-forming polymers such as the acrylic polymers described above. Sucha depression in T_(g) also can result in poor powder stability.

[0008] Moreover, the use of conventional aminoplast resins in powdercoating compositions can result in the phenomenon commonly referred toas “gassing”. “Gassing” occurs as a result of vaporization of thealcohol generated in the thermally induced aminoplast crosslinkingreaction. The alcohol vapor is driven off through the coating film uponheating and, as the viscosity of the coating increases during the curingprocess, pinholes or craters are formed as the gas escapes through thecoating surface.

[0009] Carbamate functional polymers, that is, polymers having reactivependent and/or terminal carbamate functional groups, are well known inthe art as suitable film-forming resins for liquid coating systemswhere, for example, when combined with an aminoplast curing agent, theyprovide coatings having excellent acid etch resistance. The carbamate NHgroups react readily with the methoxyl groups of the aminoplast resin,thereby forming a urethane linkage which provides this acid etchresistance. These carbamate functional polymers further provide coatingsthat have excellent durability and adhesion properties.

[0010] Copending U.S. patent application Ser. No. 09/538,836 disclosespowder coating compositions comprising a solid particulate mixture of acarbamate functional polymer, for example an acrylic, polyester and/orpolyurethane polymer, in conjunction with a glycoluril resin. Due to thehigh glass transition temperature of the carbamate functional polymer,the powder coating compositions provide improved storage stability aswell as coatings having excellent acid etch resistance. However, asdiscussed above, in some powder coating systems, the glycolurilcrosslinking agent can depress the T_(g) sufficiently to adverselyaffect powder stability.

[0011] It would, therefore, be advantageous to provide anaminoplast-based crosslinking agent suitable for use in a powder coatingcomposition which gives a highly stable powder as well as an acid etchresistant coating free of pinholes or crater resulting from “gassing”during the curing process.

SUMMARY OF THE INVENTION

[0012] In accordance with the present invention, an aminoplast-basedcrosslinking agent is provided, comprising an ungelled reaction productof the following reactants:

[0013] (A) at least one aminoplast resin;

[0014] (B) a reactive urethane group-containing adduct; and

[0015] (C) at least one compound different from (B) having activehydrogen groups reactive with aminoplast resin (A). The compound (C) isselected from at least one of:

[0016] (i) compounds having the following structure (I):

[0017]  wherein X is aromatic; R¹, R², and R³ can be the same ordifferent and each independently represents H, (cyclo)alkyl having from1 to 12 carbon atoms, aryl, alkaryl, aralkyl, or an activehydrogen-containing group, provided that at least one of R¹, R², and R³represents an active hydrogen-containing group which is reactive withthe aminoplast resin (A); and

[0018] (ii) compounds having the following structure (II or III):

[0019]  where R′ and R″ are the same or different and each independentlyrepresents an aromatic group or an alkyl group having 1 to 12 carbonatoms; and

[0020] (iii) compounds different from both (i) and (ii) and having amelting point of at least 80° C.

[0021] The reactive urethane group-containing reaction-product adduct(B) comprises a reaction product of (1) at least one mono-isocyanate and(2) at least one polyfunctional polymer having functional groupsreactive with the mono-isocyanate (1). The crosslinking agent isessentially free of urethane NH functionality and has a glass transitiontemperature of at least 25° C.

[0022] The present invention also provides a method for preparing thecrosslinking agent described above. The method comprises the steps of:(1) reacting (a) at least one mono-isocyanate, and (b) at least onepolyfunctional polymer having functional groups reactive with themono-isocyanate (a) in a molar ratio of reactant (a) to reactant (b)ranging from 1: 1.8 to 2.0 to form the reactive urethanegroup-containing adduct (B) described above; (2) combining the reactants(A), (B) and (C) described above in a ratio of total combined moles of(B) and (C) to moles of aminoplast resin (A) ranging from 1:1.5 to 3.2to form a reaction admixture; and (3) heating the reaction admixtureformed in step (2) to a temperature ranging from 95° C. to 135° C. for atime sufficient to form a powder crosslinking agent having a glasstransition temperature of at least 25° C. which is essentially free ofurethane NH functionality as determined by infrared spectroscopy.

[0023] Further provided is a powder coating composition comprising asolid particulate mixture of a reactive group-containing polymer havinga T_(g) of at least 30° C., and the crosslinking agent describedimmediately above.

[0024] The present invention additionally provides multilayer compositecoating compositions comprising a base coat deposited from a base coatfilm-forming composition and a top coat over at least a portion of thebase coat. The top coat is deposited from a powder top coatingcomposition comprising a solid particulate film-forming mixture of (A) apolymer containing reactive functional groups, said polymer having aglass transition temperature of at least 30° C. and (B) the crosslinkingagent described above.

[0025] Coated substrates are also provided.

DETAILED DESCRIPTION OF THE INVENTION

[0026] Other than in the operating examples, or where otherwiseindicated, all numbers expressing quantities of ingredients, reactionconditions and so forth used in the specification and claims are to beunderstood as being modified in all instances by the term “about.”Accordingly, unless indicated to the contrary, the numerical parametersset forth in the following specification and attached claims areapproximations that may vary depending upon the desired propertiessought to be obtained by the present invention. At the very least, andnot as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques.

[0027] Notwithstanding that the numerical ranges and parameters settingforth the broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contain certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements.

[0028] Also, it should be understood that any numerical range recitedherein is intended to include all sub-ranges subsumed therein. Forexample, a range of “1 to 10” is intended to include all sub-rangesbetween (and including) the recited minimum value of 1 and the recitedmaximum value of 10, that is, having a minimum value equal to or greaterthan 1 and a maximum value of equal to or less than 10.

[0029] The reactive urethane group-containing reaction products used inthe preparation of the crosslinking agent of the present inventioncontain urethane groups of the following structure (IV):

[0030] wherein R is a residue of a monoisocyanate. Each urethane NHgroup can react with a methoxyl group of the aminoplast resin and theresulting reaction product is an ungelled, solid material.

[0031] By contrast, for example, when a carbamate functionality (V) isused in place of a urethane structure (IV) to react under similarconditions with an aminoplast compound, the resulting reaction producttypically is a gelled material.

[0032] A carbamate functional group can be represented by the followingstructure (V):

[0033] The above-mentioned gelation is presumably due to reaction ofboth hydrogen atoms of the one carbamate NH₂ group with aminoplastmethoxyl groups.

[0034] As used herein, by “ungelled” is meant that the reaction productcan be dissolved in a suitable solvent or resin and has an intrinsicviscosity when so dissolved. The intrinsic viscosity of the reactionproduct is an indication of its molecular weight. A gelled reactionproduct, on the other hand, since it is of essentially infinitely highmolecular weight, will have an intrinsic viscosity too high to measure.Moreover, the reaction product can be melted, solidified and remelted.

[0035] The aminoplast compounds (A) useful in the preparation of thecrosslinking agent of the present invention include aminoplast resinssuch as the (alkoxyalkyl) aminotriazine compounds derived from melamine,glycoluril, benzoguanamine, acetoguanamine, formoguanamine,spiroguanamine and the like.

[0036] Aminoplast resins are based on the condensation products offormaldehyde, with an amino- or amido-group carrying substance.Condensation products obtained from the reaction of alcohols andformaldehyde with melamine, urea or benzoguanamine are most common andpreferred herein. However, condensation products of other amines andamides can also be employed, for example, aldehyde condensates oftriazines, diazines, triazoles, guanadines, guanamines and alkyl- andaryl-substituted derivatives of such compounds, including alkyl- andaryl-substituted ureas and alkyl- and aryl-substituted melamines. Someexamples of such compounds are N,N′-dimethyl urea, benzourea,dicyandiamide, formaguanamine, acetoguanamine, glycoluril, ammeline,2-chloro-4,6-diamino-1,3,5-triazine,6-methyl-2,4-diamino-1,3,5-triazine, 3,5-diaminotriazole,triaminopyrimidine, 2-mercapto4,6-diaminopyrimidine and3,4,6-tris(ethylamino)-1,3,5triazine.

[0037] While the aldehyde employed is most often formaldehyde, othersimilar condensation products can be made from other aldehydes, such asacetaldehyde, crotonaldehyde, acrolein, benzaldehyde, furfural andglyoxal.

[0038] The aminoplast resins can contain methylol or other alkylolgroups, and in most instances, at least a portion of these alkylolgroups are etherified by a reaction with an alcohol. Any monohydricalcohol can be employed for this purpose, including such alcohols asmethanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol andothers, as well as, benzyl alcohol and other aromatic alcohols, cyclicalcohols such as cyclohexanol, monoethers of glycols, andhalogen-substituted or other substituted alcohols, such as3-chloropropanol and butoxyethanol. Commonly employed aminoplast resinsare substantially alkylated with methanol or butanol.

[0039] In one embodiment of the present invention, the aminoplast resinscomprise highly alkylated, low imino aminoplast resins which have adegree of polymerization (“DP”) of less than 2.0, often less than 1.8,and typically less than 1.5. Generally, the number average degree ofpolymerization is defined as the average number of structural units perpolymer chain (see George Odian, Principles of Polymerization, JohnWiley & Sons (1991)). For purposes of the present invention, a DP of 1.0would indicate a complete monomeric triazine structure, while a DP of2.0 indicates two triazine rings joined by a methylene or methylene-oxybridge. It should be understood that the DP values reported herein andin the claims represent average DP values as determined by gelpermeation chromatography.

[0040] In one embodiment of the present invention, the aminoplastcompound comprises an (alkoxyalkyl) aminotriazine having one or lessnon-alkylated NH bond per triazine ring. An example of such anaminoplast compound is (methoxymethyl) aminotriazine. Other usefulaminoplast compounds include alkoxylated aldehyde condensates ofglycoluril and tetramethoxy methylglycoluril. Still other suitableaminoplast compounds specifically include modified melamine-formaldehyderesin, for example RESIMENE® CE-7103 commercially available fromSolutia, Inc. and CYMEL® 300; ethylated-methylatedbenzoguanamine-formaldehyde resin, for example, CYMEL® 1123; andmethylated-butylated melamine-formaldehyde resin, for example CYMEL® 11135, commercially available from Cytec Industries, Inc.

[0041] The reactive urethane group-containing adduct (B) used to preparethe crosslinking agent of the present invention comprises the reactionproduct of (1) at least one mono-isocyanate and (2) at least onepolyfunctional polymer having functional groups such as hydroxyl, amino,and thiol, reactive with the mono-isocyanate (1).

[0042] The mono-isocyanate (1) can be any of a variety of isocyanatecompounds which are monofunctional with respect to the NCO group.Non-limiting examples of suitable mono-isocyanates include thoseselected from cyclohexyl isocyanate, phenyl isocyanate, butyl isocyanateand mixtures thereof, with cyclohexyl and phenyl isocyanates beingpreferred.

[0043] The polyfunctional polymer (2) can be any of a variety ofpolymers having two or more functional groups reactive with themono-isocyanate (1). Useful polyfunctional polymers include acrylicpolymers, polyesters, polyethers and copolymers and mixtures thereof.

[0044] Suitable acrylic polymers include copolymers of one or more alkylesters of acrylic acid or methacrylic acid, optionally together with oneor more other polymerizable ethylenically unsaturated monomers. Usefulalkyl esters of acrylic acid or methacrylic acid include aliphatic alkylesters containing from 1 to 18 carbon atoms in the alkyl group.Non-limiting examples include methyl methacrylate, ethyl methacrylate,butyl methacrylate, ethyl acrylate, butyl acrylate, isobornyl acrylateand methacrylate, cyclohexyl methacrylate, and 2-ethyl hexyl acrylate.Suitable other copolymerizable ethylenically unsaturated monomersinclude vinyl aromatic compounds such as styrene and vinyl toluene;nitriles such as acrylonitrile and methacrylonitrile; vinyl andvinylidene halides such as vinyl chloride and vinylidene fluoride andvinyl esters such as vinyl acetate.

[0045] The acrylic copolymer can include hydroxyl functional groupswhich are often incorporated into the polymer by including one or morehydroxyl functional monomers in the reactants used to produce thecopolymer. Useful hydroxyl functional monomers include hydroxyalkylacrylates and methacrylates, preferably having 2 to 4 carbon atoms inthe hydroxyalkyl group, such as hydroxyethyl acrylate, hydroxypropylacrylate, 4-hydroxybutyl acrylate, hydroxy functional adducts ofcaprolactone and hydroxyalkyl acrylates, and correspondingmethacrylates.

[0046] Amino functionality may be incorporated into the acryliccopolymer by including one or more amino functional monomers in thereactants used to produce the copolymer.

[0047] Acrylic polymers can be prepared via techniques known to thoseskilled in the art.

[0048] The polyfunctional polymer can alternatively be a polyester. Suchpolymers can be prepared in a known manner by condensation of polyhydricalcohols and polycarboxylic acids. Suitable polyhydric alcohols includeethylene glycol, propylene glycol, butylene glycol, 1,6-hexylene glycol,neopentyl glycol, diethylene glycol, glycerol, trimethylol propane andpentaerythritol. Suitable polycarboxylic acids include succinic acid,adipic acid, azelaic acid, sebacic acid, maleic acid, fumaric acid,phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid andtrimellitic acid. Besides the polycarboxylic acids mentioned above,functional equivalents of the acids such as anhydrides where they existor lower alkyl esters of the acids such as methyl esters can be used.The polyesters contain a portion of free hydroxyl groups, which areavailable for reaction with the mono-isocyanate.

[0049] Preferably, the polyester comprises a condensation reactionproduct of (a) a cycloaliphatic polyol and (b) a cyclic polycarboxylicacid or anhydride.

[0050] The cycloaliphatic polyol (a) can be any of a variety ofpolyhydric cycloaliphatic compounds well known in the art. Suitableexamples of cycloaliphatic polyols include those selected from the groupconsisting of hydrogenated Bisphenol A, hydrogenated Bisphenol F,hydrogenated Bisphenol E, M, P, Z, etc. and the like, cyclohexanedimethanol, cyclohexane diol and mixtures thereof. HydrogenatedBisphenol A is preferred.

[0051] The cyclic polycarboxylic acid or anhydride (b) used to preparethe polyester can be any cyclic compound having two or more carboxylicacid/anhydride groups per molecule. Preferably, the cyclicpolycarboxylic acid/anhydride (b) is selected from the group consistingof hexahydrophthalic acid, phthalic acid, isophthalic acid, terephthalicacid, anhydrides thereof, and mixtures thereof. Hexahydrophthalicanhydride is preferred.

[0052] The polyester can be prepared by general condensation reactiontechniques well-known in the art so long as the ratio of reactants andreaction conditions are such that the resulting condensation reactionproduct comprises two or more reactive hydroxyl groups per molecule. Inthe preferred embodiment wherein the polyester comprises thecondensation reaction product of (a) a cycloaliphatic polyol and (b) acyclic polycarboxylic acid or anhydride, the molar ratio of thecycloaliphatic polyol (a) to the cyclic polycarboxylic acid or anhydride(b) typically ranges from 1.5 to 2.5:1, preferably from 1.7 to 2.3:1,and more preferably from 1.8 to 2.2:1.

[0053] Polyethers such as polypropylene glycol can also be used aspolyfunctional polymers preparing the reactive urethane group-containingadduct (B).

[0054] The polyfunctional polymer (2) often has a number averagemolecular weight (Mn) ranging from 300 to 3000, usually from 350 to2500, and typically from 400 to 2200. Unless stated otherwise, as usedin the specification and in the claims, molecular weights are numberaverage molecular weights for polymeric materials indicated as “Mn” andobtained by gel permeation chromatography using a polystyrene standardin an art-recognized manner.

[0055] The hydroxyl value of the polyfunctional polymer (2) often isgreater than 90, usually ranges from 100 to 180, and typically from 110to 170.

[0056] The polyfunctional polymer (2) often has a glass transitiontemperature of at least 30° C., usually at least 35° C., and typicallyat least 40° C. The T_(g) of the polyfunctional polymer (2) can becalculated or it can be measured experimentally using differentialscanning calorimetry (rate of heating 10° C. per minute, T_(g) taken atthe first inflection point). Unless otherwise indicated, the statedT_(g) as used herein refers to the measured T_(g).

[0057] The reactive urethane group-containing adduct (B) can be preparedby general NCO/OH reaction techniques well-known in the art, so long asthe ratio of reactants (1) and (2) and reaction conditions are such thatthe resulting reaction product comprises at least one, typically atleast two, reactive urethane NH groups per molecule. For purposes of thepresent invention, the molar ratio of the polyfunctional polymer (2) tothe mono-isocyanate (1) can range from 1:1.7 to 2.0, usually from 1:1.75to 2.0, and typically from 1:1.8 to 2.0.

[0058] The reactive urethane group-containing adduct (B) is preparedgenerally as follows. Typically, the polyfunctional polymer is dissolvedin an appropriate aromatic solvent, such as xylene or toluene, with atin compound, for example dibutyl tin dilaurate and dibutyl tindiacetate, as a catalyst. The mixture is then preheated to approximately55° C. and the mono-isocyanate is added dropwise. The addition ratetypically is adjusted so that the reaction temperature is less than 90°C. The reaction is complete when all the isocyanate functionality isconsumed.

[0059] As aforementioned, in addition to the aminoplast resin (A) andthe reactive urethane group-containing adduct (B) described immediatelyabove, the reactants used to form the crosslinking agent of the presentinvention further comprise as component (C) at least one compounddifferent from (B) having active hydrogen groups reactive withaminoplast resin (A). As previously discussed, compound (C) is selectedfrom at least one of (i) compounds having the following structure (I):

[0060] wherein X is aromatic; R¹, R², and R³ can be the same ordifferent and each independently represents H, (cyclo)alkyl having from1 to 12 carbon atoms, aryl, alkaryl, aralkyl, or an activehydrogen-containing group which is reactive with the aminoplast resin(A), provided that at least one of R¹, R², and R³ represents an activehydrogen-containing group which is reactive with the aminoplast resin(A); (ii) compounds having the following structure (II or III):

[0061] where R′ and R″ are the same or different and each independentlyrepresents an aromatic group or an alkyl group having 1 to 12 carbonatoms; and (iii) compounds different from both (i) and (ii) and having amelting point of at least 80° C. Mixtures of compounds (i), (ii) and(iii) can be used. As used herein, by “(cyclo)alkyl” is meant both alkyland cycloalkyl groups.

[0062] In one embodiment of the present invention, compound (C)comprises at least one of compound (i) having the previously describedstructure (I). As aforementioned, the substituent group X represents anaromatic, for example, phenyl, and substituted phenyl groups, or acycloaliphatic group, for example, cyclohexyl. These groups can be anyfused or bridged ring structures such as naphthyl, anthracyl, andbenzofuranyl. Also, the aromatic groups can be unsubstituted orsubstituted with heteroatoms, for example O, N and S. Non-limitingexamples of aromatic groups suitable as the substituent include phenyl,naphthyl, anthracyl, pyrene, benzofuranyl, and the like.

[0063] As previously mentioned, at least one of R¹, R², and R³represents a group comprising an active hydrogen-containing groupreactive with the aminoplast resin (A) such as a group selected fromhydroxyl, amide, amine, carboxylic acid, carbamate, urea, thiol, andcombinations thereof. In one embodiment of the present invention,compound (C) comprises at least one compound having the structure (I)above wherein at least one of R¹, R², and R³ represents a groupcomprising at least one hydroxyl group. Nonlimiting examples of activehydrogen-containing compounds suitable for use as the reactant (c)(i)include benzyl alcohol and substituted benzyl alcohols such as3-phenoxybenzyl alcohol and 4-methoxybenzyl alcohol, phenethyl alcohol,benzopinacol, N-benzylformamide, benzyl lactate, benzyl mandelate,benzyl mercaptan, N-benzylmethamine, 3-furanmethanol, furfuryl alcohol,pyridylcarbinols, for example, 2-pyridylcarbinol, and 3-pyridylcarbinol,1-pyrenemethanol, 9-anthrancenemethanol, 9-fluorenemethanol,9-hydroxyfluorene, 9-hydroxyxanthene, 9-phenylxanthen-9-ol,4-stilbenemethanol and triphenylmethanol.

[0064] In another embodiment of the present invention, the activehydrogen-containing compound (C) (ii) comprises compounds having thefollowing structure (II):

[0065] or dimer derivatives thereof as discussed below, where R′ and R″are the same or different and each independently represents an aromaticgroup or an alkyl group having 1 to 12 carbon atoms. In one embodimentof the present invention, one or both of the substituent groups R′ andR″ are aromatic groups, for example phenyl, naphthyl, methoxy phenyl,and dimethylaminophenyl groups.

[0066] Also, suitable aromatic groups can contain one or moreheteroatoms, such as O, N and S, either internal or external to thearomatic ring. The heteroatoms external to the ring may be attacheddirectly to the ring or indirectly through one or more carbon atoms. Oneor more heteroatoms may be present in each such substituent and one ormore substituents may be attached to the aromatic ring. The heteroatomcontaining substituent group(s) may be attached to the aromatic ring inany position or combination of positions on the ring. Suitableheteroatomic substituent groups include but are not limited to amines,ethers, esters, ketones, amides, halides, sulfonamides, nitro andcarboxylic acid groups. Heteroatoms internal to the aromatic ring may bepresent in any position or combination of positions. For example, suchheteroaromatic groups can include but are not limited to furans,pyridines, thiophenes, triazines, imidazoles, oxazoles, thiazoles,pyrazoles and triazoles. Non-limiting examples of such compounds includeanisoin, pyridoin, furoin, bufyroin.

[0067] In one particular embodiment of the present invention, the activehydrogen-containing compound (c)(ii) comprises an activehydrogen-containing compound selected from benzoin, hydroxycyclohexylphenylketone, and mixtures thereof.

[0068] Compounds having the general structure (II) above are known toform dimeric derivatives, particularly when R′ and R″ are alkyl (MerckIndex, 11ed, p 10, 55).

[0069] Such dimer derivatives can have the structure (III) above whereR′ and R″ are as described above for the structure (II).

[0070] In yet another embodiment of the present invention, the activehydrogen-containing compound (C) comprises at least one of compound(iii), which is different from both (i) and (ii) and has a melting pointof at least 80° C. The melting point of a compound can be determined byusing a standard capillary melting point apparatus or by thermalanalysis (ASTM E974-95).

[0071] Generally, the melting point of the active hydrogen-containingcompound (C)(iii) is less than 250° C., usually less than 220° C., andtypically less than 200° C. Also, the melting point of the activehydrogen-containing compound (C)(iii) generally is at least 80° C.,usually at least 90° C., and typically at least 100° C. The meltingpoint of the active hydrogen-containing compound (C)(iii) can rangebetween any combination of these values inclusive of the recited values.Nonlimiting examples of compounds suitable for use as reactant (C)(iii)include mono-alcohols such as borneol, norborneol, isoborneol,1-adamantanemethanol, 1-adamantanol, 2-methyl-2-adamantanol and5-norbornen-2-ol; secondary amides, such as aliphatic cyclic amides suchas 1-methylhydantoin, 2,4-thiazolidinedione, 2-azacyclotridecanone,3,4,5,6,7,8-hexahydro-2(1H)-quinoline,4-azatricyclo(4.3.1.1(3,8))undecan-5-one and 4-methoxy-3-pyrrolin-2-one;aliphatic open chain amides, such as N-(1-adamantyl)acetamide) andN-tert-butylacrylamide; aromatic (poly)cyclic amides, including lactams,such as 1-acetamidopyrene, 2-acetamide-3-nitro-9-fluorenone,2-acetoamide-7-fluorfluorene, 2-acetamidofluorene,4-acetamido-9-fluorenone, naphthol AS acetate,1-phenyl-3-pyrazolidinone,2,3-dimethyl-1-(4-methylphenyl)-3-pyrazolin-5-one,3,4-dimethyl-l-phenyl-3-pyrazolin-5-one,3-(4-ethoxyphenyl)-1-(2-nitrophenyl)-hydantoin, 4-acetamidoantipyrine,and 4-acetamidobenzaldehyde; aromatic open chain amides, such as3-acetamidocoumarin and p-acetophenetidide; and mono-urethanes such asthose obtained by reacting high melting point mono-alcohols (such asthose described immediately above) with suitable mono-isocyanates.

[0072] The active hydrogen-containing compound (C) can comprise one ormore of compounds (C)(i), (C)(ii), and (C)(iii). Inclusion of the activehydrogen-containing compound (C) as a reactant in the preparation of thecrosslinking agent of the present invention can provide severaladvantages. First, reaction of the active hydrogen-containing compound(C) with the aminoplast resin (A) can generally increase the T_(g) ofthe resultant crosslinker as compared to an analogous crosslinking agenthaving no such modification. Also, compounds such as (C)(i), (C)(ii),and (C)(iii) described above can allow for the reaction of more alkoxygroups of the aminoplast resin (A) without resulting in a gelledreaction product. Such a crosslinking agent when incorporated intocurable powder coating compositions can effect less gassing upon curing.Furthermore, when crosslinking agents of the present invention are usedin curable powder coating compositions, the degassing agent may bereleased in situ. This can reduce adverse effects, for example,yellowing of the film, which can be caused by the presence of thedegassing agent during curing processes.

[0073] In the preparation of the crosslinking agent of the presentinvention, the reactive urethane group-containing adduct (B) is preparedin a first step as described above. Secondly, the aminoplast resin (A),reactive urethane group-containing adduct (B) and activehydrogen-containing compound (C) are combined in a ratio of totalcombined moles of (B) and (C) to moles of aminoplast resin (A) rangingfrom 1:1.5 to 3.2 to form a reaction admixture. With regard to theaminoplast resin (A), it should be understood that the theoreticalmonomeric molecular weight of the aminoplast resin (that is, DP=1) isused to calculate the above-referenced “molar ratio”. The reactants aretypically combined in a suitable aromatic solvent, for example, xyleneand toluene, together with an appropriate strong acid catalyst.Non-limiting examples of suitable strong acid catalysts include dodecylbenzene sulfonic acid and para-toluene sulfonic acid. Then the reactionadmixture formed in the second step is heated to a temperature rangingfrom 95° C. to 135° C. for a time sufficient to form a solidcrosslinking agent having a glass transition temperature of at least 25°C. This results in a stable crosslinking agent that is essentially freeof urethane NH functionality. The reaction is monitored via infraredspectroscopy or other suitable analytical means for the disappearance ofurethane NH functionality relative to an internal standard (i.e., thesignal of a structure that will remain unchanged during the reaction,for example, the urethane carbonyl signal). The reaction is typicallyterminated when this end point is detected by infrared spectroscopy orother suitably analytical methods.

[0074] The crosslinking agent of the present invention can have a glasstransition temperature of at least 25° C., usually at least 30° C.,often at least 35° C., and typically at least 40° C. Also, thecrosslinking agent can have a glass transition temperature less than150° C., usually less than 120° C., often less than 100° C., andtypically less than 80° C. The glass transition temperature of thecrosslinking agent can range between any combination of these values,inclusive of the recited values.

[0075] The present invention also relates to a curable compositioncomprising (1) a polymer containing reactive functional groups and (2) acrosslinking agent having functional groups reactive with the functionalgroups of the polymer (1). The crosslinking agent (2) can comprise thecrosslinking agent described above.

[0076] In an embodiment of the present invention, the curablecomposition is a powder coating composition comprising a solidparticulate film-forming mixture of (1) a polymer containing reactivefunctional groups and having a glass transition temperature of at least30° C.; e.g., a hydroxyl and/or an epoxide functional acrylic polymer,and (2) a crosslinking agent comprising the crosslinking agent describedabove, having functional groups reactive with the functional groups ofthe polymer (1).

[0077] Curable powder coatings are particulate compositions that aresolid and free flowing at ambient room temperature. The components (1)and (2) of the curable powder coating composition may each independentlycomprise one or more functional species, and are each present in amountssufficient to provide cured coatings having a desirable combination ofphysical properties, e.g., smoothness, optical clarity, scratchresistance, solvent resistance and hardness.

[0078] As used herein, the term “reactive” refers to a functional groupthat forms a covalent bond with another functional group under suitablereaction conditions.

[0079] As used herein, the term “cure” as used in connection with acomposition, e.g., “a curable composition,” shall mean that anycrosslinkable components of the composition are at least partiallycrosslinked. In certain embodiments of the present invention, thecrosslink density of the crosslinkable components, i.e., the degree ofcrosslinking, ranges from 5% to 100% of complete crosslinking. In otherembodiments, the crosslink density ranges from 35% to 85% of fullcrosslinking. In other embodiments, the crosslink density ranges from50% to 85% of full crosslinking. One skilled in the art will understandthat the presence and degree of crosslinking, i.e., the crosslinkdensity, can be determined by a variety of methods, such as dynamicmechanical thermal analysis (DMTA) using a Polymer Laboratories MK IIIDMTA analyzer conducted under nitrogen. This method determines the glasstransition temperature and crosslink density of free films of coatingsor polymers. These physical properties of a cured material are relatedto the structure of the crosslinked network.

[0080] According to this method, the length, width, and thickness of asample to be analyzed are first measured, the sample is tightly mountedto the Polymer Laboratories MK III apparatus, and the dimensionalmeasurements are entered into the apparatus. A thermal scan is run at aheating rate of 3° C./min, a frequency of 1 Hz, a strain of 120%, and astatic force of 0.01N, and sample measurements occur every two seconds.The mode of deformation, glass transition temperature, and crosslinkdensity of the sample can be determined according to this method. Highercrosslink density values indicate a higher degree of crosslinking in thecoating.

[0081] Also, as used herein, in the specification and in the claims, theterm “polymer” is intended to refer to oligomers and both homopolymersand copolymers.

[0082] The polymer (1) can be any of a variety of polymers havingaminoplast-reactive functional groups as are well known in the art, solong as the T_(g) of the polymer is sufficiently high to permit theformation of a stable, solid particulate composition. The T_(g) of thepolymer (1) typically is at least 30° C., preferably at least 40° C.,more preferably at least 50° C. The T_(g) of the polymer (1) alsotypically is less than 130° C., preferably less than 100° C., morepreferably less than 80° C. The T_(g) of the functional group-containingpolymer (1) can range between any combination of these values inclusiveof the recited values.

[0083] Non-limiting examples of polymers having reactive functionalgroups useful in the powder coating compositions of the invention as thepolymer (1) include those selected from acrylic, polyester, polyepoxide,polyurethane and polyether polymers. Acrylic and polyester polymers aremost often employed.

[0084] The polymer (1) can comprise a wide variety of reactivefunctional groups, for example hydroxyl, carboxyl, carbamate, epoxyand/or amide functional groups. The polymer (1) preferably comprisesreactive functional groups selected the group consisting of hydroxyl,epoxy, carboxyl and/or carbamate functional groups. In one preferredembodiment, the polymer (1) comprises hydroxyl and/or carbamatefunctional groups. In another preferred embodiment of the presentinvention, the polymer (1) comprises hydroxyl and/or epoxy functionalgroups.

[0085] Suitable functional group-containing acrylic polymers includecopolymers prepared from one or more alkyl esters of acrylic acid ormethacrylic acid and, optionally, one or more other polymerizableethylenically unsaturated monomers. Suitable alkyl esters of acrylic ormethacrylic acid include methyl methacrylate, ethyl methacrylate, butylmethacrylate, ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate.Also, when epoxy functional polymers are desired, epoxy functionalmonomers, for example glycidyl acrylate and glycidyl methacrylate andallyl glycidyl ether, are suitable. Ethylenically unsaturated carboxylicacid functional monomers, for example acrylic acid and/or methacrylicacid, can also be used when a carboxylic acid functional acrylic polymeris desired. Amide functional acrylic polymers can be formed bypolymerizing ethylenically unsaturated acrylamide monomers, such asN-butoxymethyl acrylamide and N-butoxyethyl acrylamide with otherpolymerizable ethylenically unsaturated monomers. Non-limiting examplesof suitable other polymerizable ethylenically unsaturated monomersinclude vinyl aromatic compounds, such as styrene and vinyl toluene;nitriles, such as acrylonitrile and methacrylonitrile; vinyl andvinylidene halides, such as vinyl chloride and vinylidene fluoride andvinyl esters, such as vinyl acetate.

[0086] In one embodiment of the present invention, the acrylic polymerscontain hydroxyl functionality which can be incorporated into theacrylic polymer through the use of hydroxyl functional monomers such ashydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylateand hydroxypropyl methacrylate which may be copolymerized with the otheracrylic monomers mentioned above.

[0087] In another embodiment of the invention, the acrylic polymer canbe prepared from ethylenically unsaturated, beta-hydroxy esterfunctional monomers. Such monomers are derived from the reaction of anethylenically unsaturated acid functional monomer, such asmonocarboxylic acids, for example, acrylic acid, and an epoxy compoundwhich does not participate in the free radical initiated polymerizationwith the unsaturated acid monomer. Examples of such epoxy compounds areglycidyl ethers and esters. Suitable glycidyl ethers include glycidylethers of alcohols and phenols, such as butyl glycidyl ether, octylglycidyl ether, phenyl glycidyl ether and the like. Suitable glycidylesters include those which are commercially available from ShellChemical Company under the tradename CARDURA® E; and from Exxon ChemicalCompany under the tradename GLYDEXX®-10.

[0088] Alternatively, the beta-hydroxy ester functional monomers areprepared from an ethylenically unsaturated, epoxy functional monomer,for example glycidyl methacrylate and allyl glycidyl ether, and asaturated carboxylic acid, such as a saturated monocarboxylic acid, forexample, isostearic acid.

[0089] The hydroxyl group-containing acrylic polymers useful in thecompositions of the present invention typically have a hydroxyl valueranging from 10 to 150, preferably from 15 to 100, and more preferablyfrom 20 to 50.

[0090] The acrylic polymer is typically prepared by solutionpolymerization techniques in the presence of suitable initiators such asorganic peroxides or azo compounds, for example, benzoyl peroxide orN,N-azobis(isobutyronitrile). The polymerization can be carried out inan organic solution in which the monomers are soluble by techniquesconventional in the art.

[0091] Pendent and/or terminal carbamate functional groups can beincorporated into the acrylic polymer by copolymerizing the acrylicmonomer with a carbamate functional vinyl monomer, such as a carbamatefunctional alkyl ester of methacrylic acid. These carbamate functionalalkyl esters are prepared by reacting, for example, a hydroxyalkylcarbamate, such as the reaction product of ammonia and ethylenecarbonate or propylene carbonate, with methacrylic anhydride. Othercarbamate functional vinyl monomers can include the reaction product ofhydroxyethyl methacrylate, isophorone diisocyanate and hydroxypropylcarbamate. Still other carbamate functional vinyl monomers may be used,such as the reaction product of isocyanic acid (HNCO) with a hydroxylfunctional acrylic or methacrylic monomer such as hydroxyethyl acrylate,and those carbamate functional vinyl monomers described in U.S. Pat. No.3,479,328.

[0092] Carbamate groups can also be incorporated into the acrylicpolymer by a “transcarbamoylation” reaction in which a hydroxylfunctional acrylic polymer is reacted with a low molecular weightcarbamate derived from an alcohol or a glycol ether. The carbamategroups exchange with the hydroxyl groups yielding the carbamatefunctional acrylic polymer and the original alcohol or glycol ether.

[0093] The low molecular weight carbamate functional material derivedfrom an alcohol or glycol ether is first prepared by reacting thealcohol or glycol ether with urea in the presence of a catalyst such asbutyl stannoic acid. Suitable alcohols include lower molecular weightaliphatic, cycloaliphatic and aromatic alcohols, such as methanol,ethanol, propanol, butanol, cyclohexanol, 2-ethylhexanol and3-methylbutanol. Suitable glycol ethers include ethylene glycol methylether and propylene glycol methyl ether. Propylene glycol methyl etheris preferred.

[0094] Also, hydroxyl functional acrylic polymers can be reacted withisocyanic acid yielding pendent carbamate groups. Note that theproduction of isocyanic acid is disclosed in U.S. Pat. No. 4,364,913.Likewise, hydroxyl functional acrylic polymers can be reacted with ureato give an acrylic polymer with pendent carbamate groups.

[0095] Epoxide functional acrylic polymers are typically prepared bypolymerizing one or more epoxide functional ethylenically unsaturatedmonomers, e.g., glycidyl (meth)acrylate and allyl glycidyl ether, withone or more ethylenically unsaturated monomers that are free of epoxidefunctionality, e.g., methyl (meth)acrylate, isobornyl (meth)acrylate,butyl (meth)acrylate and styrene. Examples of epoxide functionalethylenically unsaturated monomers that may be used in the preparationof epoxide functional acrylic polymers include, but are not limited to,glycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate,2-(3,4-epoxycyclohexyl)ethyl (meth)acrylate and allyl glycidyl ether.Examples of ethylenically unsaturated monomers that are free of epoxidefunctionality include those described above as well as those describedin U.S. Pat. No. 5,407,707 at column 2, lines 17 through 56, whichdisclosure is incorporated herein by reference. In one embodiment of thepresent invention, the epoxide functional acrylic polymer is preparedfrom a majority of (meth)acrylate monomers.

[0096] The functional group-containing acrylic polymer typically canhave a Mn ranging from 500 to 30,000 and typically from 1000 to 5000. Ifcarbamate functional, the acrylic polymer typically can have acalculated carbamate equivalent weight within the range of 15 to 150,and typically less than 50, based on equivalents of reactive carbamategroups.

[0097] Non-limiting examples of functional group-containing polyesterpolymers suitable for use as the polymer (1) in the powder coatingcompositions of the present invention can include linear or branchedpolyesters having hydroxyl, carboxyl and/or carbamate functionality.Such polyester polymers are generally prepared by the polyesterificationof a polycarboxylic acid or anhydride thereof with polyols and/or anepoxide using techniques known to those skilled in the art. Usually, thepolycarboxylic acids and polyols are aliphatic or aromatic dibasic acidsand diols. Transesterification of polycarboxylic acid esters usingconventional techniques is also possible.

[0098] The polyols usually employed in the preparation of the polyester(or the polyurethane polymer, as described below) include alkyleneglycols, such as ethylene glycol and other diols, such as neopentylglycol, hydrogenated Bisphenol A, cyclohexanediol, butyl ethyl propanediol, trimethyl pentane diol, cyclohexanedimethanol, caprolactonediol,for example, the reaction product of epsilon-caprolactone and ethyleneglycol, hydroxy-alkylated bisphenols, polyether glycols, for example,poly(oxytetramethylene) glycol and the like. Polyols of higherfunctionality can also be used. Examples include trimethylolpropane,trimethylolethane, pentaerythritol, tris-hydroxyethylisocyanurate andthe like. Branched polyols, such as trimethylolpropane, are preferred inthe preparation of the polyester.

[0099] The acid component used to prepare the polyester polymer caninclude, primarily, monomeric carboxylic acids or anhydrides thereofhaving 2 to 18 carbon atoms per molecule. Among the acids which areuseful are cycloaliphatic acids and anhydrides, such as phthalic acid,isophthalic acid, terephthalic acid, tetrahydrophthalic acid,hexahydrophthalic acid, methylhexahydrophthalic acid, 1,3-cyclohexanedicarboxylic acid and 1,4-cyclohexane dicarboxylic acid. Other suitableacids include adipic acid, azelaic acid, sebacic acid, maleic acid,glutaric acid, decanoic diacid, dodecanoic diacid and other dicarboxylicacids of various types. The polyester can include minor amounts ofmonobasic acids such as benzoic acid, stearic acid, acetic acid andoleic acid. Also, there may be employed higher carboxylic acids, such astrimellitic acid and tricarballylic acid. Where acids are referred toabove, it is understood that anhydrides thereof which exist may be usedin place of the acid. Also, lower alkyl esters of diacids such asdimethyl glutarate and dimethyl terephthalate can be used. Because it isreadily available and low in cost, terephthalic acid is preferred.

[0100] Pendent and/or terminal carbamate functional groups can beincorporated into the polyester by first forming a hydroxyalkylcarbamate which can be reacted with the polyacids and polyols used informing the polyester. The hydroxyalkyl carbamate is condensed with acidfunctionality on the polyester yielding carbamate functionality.Carbamate functional groups can also be incorporated into the polyesterby reacting a hydroxyl functional polyester with a low molecular weightcarbamate functional material via a transcarbamoylation process similarto the one described above in connection with the incorporation ofcarbamate groups into the acrylic polymers or by reacting isocyanic acidwith a hydroxyl functional polyester.

[0101] The functional group-containing polyester polymer can have a Mnranging from 500 to 30,000, typically about 1000 to 5000. If carbamatefunctional, the polyester polymer can have a calculated carbamateequivalent weight within the range of 15 to 150, typically 20 to 75,based on equivalents of reactive pendent or terminal carbamate groups.

[0102] Epoxide functional polyesters can be prepared by art-recognizedmethods, which typically include first preparing a hydroxy functionalpolyester that is then reacted with epichlorohydrin. Polyesters havinghydroxy functionality may be prepared by art-recognized methods, whichinclude reacting carboxylic acids (and/or esters thereof) having acid(or ester) functionalities of at least 2, and polyols having hydroxyfunctionalities of at least 2. As is known to those of ordinary skill inthe art, the molar equivalents ratio of carboxylic acid groups tohydroxy groups of the reactants is selected such that the resultingpolyester has hydroxy functionality and the desired molecular weight.

[0103] Non-limiting examples of suitable polyurethane polymers havingpendent and/or terminal hydroxyl and/or carbamate functional groupsinclude the polymeric reaction products of polyols, which are preparedby reacting the polyester polyols or acrylic polyols, such as thosementioned above, with a polyisocyanate such that the OH/NCO equivalentratio is greater than 1:1 such that free hydroxyl groups are present inthe reaction product. Such reactions employ typical conditions forurethane formation, for example, temperatures of 60° C. to 90° C. and upto ambient pressure, as known to those skilled in the art.

[0104] The organic polyisocyanates which can be used to prepare thefunctional group-containing polyurethane polymer include aliphatic oraromatic polyisocyanates or a mixture of the two. Diisocyanates are mostoften used, although higher polyisocyanates can be used in place of orin combination with diisocyanates.

[0105] Examples of suitable aromatic diisocyanates include4,4′-diphenylmethane diisocyanate and toluene diisocyanate. Examples ofsuitable aliphatic diisocyanates include straight chain aliphaticdiisocyanates, such as 1,6-hexamethylene diisocyanate. Also,cycloaliphatic diisocyanates can be employed. Examples includeisophorone diisocyanate and 4,4′-methylene-bis-(cyclohexyl isocyanate).Examples of suitable higher polyisocyanates include 1,2,4-benzenetriisocyanate and polymethylene polyphenyl isocyanate.

[0106] Terminal and/or pendent carbamate functional groups can beincorporated into the polyurethane by reacting a polyisocyanate with apolyester polyol containing the terminal/pendent carbamate groups.Alternatively, carbamate functional groups can be incorporated into thepolyurethane by reacting a polyisocyanate with a polyester polyol and ahydroxyalkyl carbamate or isocyanic acid as separate reactants.Carbamate functional groups can also be incorporated into thepolyurethane by reacting a hydroxyl functional polyurethane with a lowmolecular weight carbamate functional material via a transcarbamoylationprocess similar to the one described above in connection with theincorporation of carbamate groups into the acrylic polymer.

[0107] The hydroxyl and/or carbamate functional group-containingpolyurethane polymers can have a Mn ranging from 500 to 20,000,typically from 1000 to 5000. If carbamate functional, the polyurethanepolymer can have a carbamate equivalent weight within the range of 15 to150, typically 20 to 75, based on equivalents of reactive pendent orterminal carbamate groups.

[0108] For some applications it may be desirable to employ a functionalgroup-containing polyether polymer in the powder coating compositions ofthe present invention. Suitable hydroxyl and/or carbamate functionalpolyether polymers can be prepared by reacting a polyether polyol withurea under reaction conditions well known to those skilled in the art.More often, the polyether polymer is prepared by a transcarbamoylationreaction similar to the reaction described above in connection with theincorporation of carbamate groups into the acrylic polymers.

[0109] Examples of polyether polyols are polyalkylene ether polyolswhich include those having the following structural formulae (VI) and(VII):

[0110] where the substituent R⁴ is hydrogen or lower alkyl containingfrom 1 to 5 carbon atoms including mixed substituents, n is typicallyfrom 2 to 6, and m is from 8 to 100 or higher. Note that the hydroxylgroups, as shown in structures (VI) and (VII) above, are terminal to themolecules. Included are poly(oxytetramethylene) glycols,poly(oxytetraethylene) glycols, poly(oxy-1,2-propylene) glycols andpoly(oxy-1,2-butylene) glycols.

[0111] Also useful are polyether polyols formed from oxyalkylation ofvarious polyols, for example, diols, such as ethylene glycol,1,6-hexanediol, Bisphenol A and the like, or other higher polyols, suchas trimethylolpropane, pentaerythritol and the like. Polyols of higherfunctionality which can be utilized as indicated can be made, forinstance, by oxyalkylation of compounds, such as sucrose or sorbitol.One commonly utilized oxyalkylation method is reaction of a polyol withan alkylene oxide, for example, propylene or ethylene oxide, in thepresence of a conventional acidic or basic catalyst as known to thoseskilled in the art. Typical oxyalkylation reaction conditions may beemployed. Suitable polyethers include those sold under the namesTERATHANE® and TERACOL®, available from E. I. Du Pont de Nemours andCompany, Inc. and POLYMEG®, available from Q O Chemicals, Inc., asubsidiary of Great Lakes Chemical Corp.

[0112] Epoxide functional polyethers can be prepared from a hydroxyfunctional monomer, e.g., a diol, and an epoxide functional monomer,and/or a monomer having both hydroxy and epoxide functionality. Suitableepoxide functional polyethers include, but are not limited to, thosebased on 4,4′-isopropylidenediphenol (Bisphenol A), a specific exampleof which is EPON® RESIN 2002 available commercially from ShellChemicals.

[0113] Suitable functional group-containing polyether polymers can havea number average molecular weight (Mn) ranging from 500 to 30,000 andtypically from 1000 to 5000. If carbamate functional, the polyetherpolymers can have a carbamate equivalent weight of within the range of15 to 150, typically 25 to 75, based on equivalents of reactive pendentand/or terminal carbamate groups and the solids of the polyetherpolymer.

[0114] It should be understood that the carbamate functionalgroup-containing polymers often contain residual hydroxyl functionalgroups which provide additional crosslinking sites. Suchcarbamate/hydroxyl functional group-containing polymer (1) can have aresidual hydroxyl value ranging from 0.5 to 10, more preferably from 1to 10, and even more preferably from 2 to 10 (mg KOH per gram).

[0115] It should be noted that any of the functional group-containingpolymers described above may be used as the polyfunctional polymer (2)in the preparation of the urethane group-containing adduct previouslydescribed, provided the polymer has functional groups reactive withmono-isocyanates.

[0116] The functional group-containing polymer (1) can be present in thepowder coating composition of the present invention in an amount rangingfrom at least 5 percent by weight, usually at least 20 percent byweight, often at least 30 percent by weight, and typically at least 40percent by weight based on the total weight of resin solids in thefilm-forming composition. The functional group-containing polymer (1)also can be present in the powder coating compositions of the presentinvention in an amount less than 95 percent by weight, usually less than90 percent by weight, often less than 85 percent by weight, andtypically less than 80 percent by weight based on the total weight ofthe powder coating composition. The amount of the functionalgroup-containing polymer (1) present in the powder coating compositionsof the present invention can range between any combination of thesevalues inclusive of the recited values.

[0117] As mentioned above, the powder coating compositions of thepresent invention further comprise, as component (2), the crosslinkingagent described in detail above. The crosslinking agent (2) can bepresent in the powder coating compositions of the present invention inan amount ranging from at least 5 percent by weight, often at least 10percent by weight, often at least 15 percent by weight, and typically atleast 20 percent by weight based on the total weight of the powdercoating composition. The crosslinking agent (2) also can be present inthe powder coating compositions of the present invention in an amountless than 95 percent by weight, usually less than 80 percent by weight,often less than 70 percent by weight, and typically less than 60 percentby weight based on the total weight of the powder coating composition.The amount of the crosslinking agent (2) present in the powder coatingcompositions of the present invention can range between any combinationof these values inclusive of the recited values.

[0118] If desired, the powder coating compositions of the presentinvention also can include one or more adjuvant curing agents differentfrom the crosslinking agent (2). The adjuvant curing agent can be anycompound having functional groups reactive with the functional groups ofthe polymer (1) or the crosslinking agent (2) described immediatelyabove. Non-limiting examples of suitable adjuvant curing agents includeblocked isocyanates, triazine compounds, glycoluril resins, and mixturesthereof.

[0119] The blocked isocyanates suitable for use as the adjuvant curingagent in the powder coating compositions of the invention are knowncompounds and can be obtained from commercial sources or may be preparedaccording to published procedures. Upon being heated to cure the powdercoating compositions, the isocyanates are unblocked and the isocyanategroups become available to react with the functional groups of thepolymer (1).

[0120] Any suitable aliphatic, cycloaliphatic or aromatic alkylmonoalcohol known to those skilled in the art can be used as a blockingagent for the isocyanate. Other suitable blocking agents include oximesand lactams. Non-limiting examples of suitable blocked isocyanate curingagents include those based on isophorone diisocyanate blocked withepsilon-caprolactam; toluene 2,4-toluene diisocyanate blocked withepsilon-caprolactam; or phenol-blocked hexamethylene diisocyanate. Theblocked isocyanates mentioned immediately above are described in detailin U.S. Pat. No. 4,988,793 at column 3, lines 1 to 36. Preferred blockedisocyanate curing agents include BF-1530, which is the reaction productof epsilon-caprolactam blocked T1890, a trimerized isophoronediisocyanate (“IPDI”) with an isocyanate equivalent weight of 280, andBF-1540, a uretidione of IPDI with an isocyanate equivalent weight of280, all of which are available from Creanova of Somerset N.J.

[0121] Conventional aminoplast crosslinkers can be used as the adjuvantcuring agent provided that the T_(g) of the coating is not lowered to anundesirable extent. Such aminoplast resins can include aldehydecondensates of glycoluril, such as those described above. Glycolurilresins suitable for use as the adjuvant curing agent in the powdercoating compositions of the invention include POWDERLINK® 1174,commercially available from Cytec Industries, Inc. of Stamford, Conn.

[0122] When employed, the adjuvant curing agent can be present in thepowder coating compositions of the present invention in an amountranging from 10 to 0.5 percent by weight, usually from 10 to 1 percentby weight, often from 5 to 2 percent by weight, and typically from 4 to2 percent by weight based on the total weight of the powder coatingcomposition.

[0123] Also suitable for use as an adjuvant curing agent in the powdercoating compositions of the present invention are triazine compounds,such as the tricarbamoyl triazine compounds described in detail in U.S.Pat. No. 5,084,541. When used, the triazine curing agent is typicallypresent in the powder coating composition of the present invention in anamount ranging up to about 20 percent by weight, and preferably fromabout 1 to 20 percent by weight, percent by weight based on the totalweight of the powder coating composition. Mixtures of theabove-described curing agents also can be used advantageously.

[0124] Also, it should be understood that for purposes of the presentinvention, the curable powder coating compositions which contain epoxygroup-containing polymers also typically include an epoxide-reactivecuring (i.e., crosslinking) agent, preferably an acid functional curingagent, in addition to the crosslinking agent (2). A secondary hydroxylgroup can be generated upon reaction of each epoxy functional group witha functional group of the epoxide-reactive curing agent. These secondaryhydroxyl groups are then available for subsequent reaction with theaminoplast-based crosslinking agent of the present invention.

[0125] Epoxide-reactive curing agents which can be used in curablepowder coating compositions comprising an epoxide functional polymer mayhave functional groups selected from hydroxyl, thiol, primary amines,secondary amines, acid (e.g. carboxylic acid) and mixtures thereof.Useful epoxide reactive curing agents having amine functionalityinclude, for example, dicyandiamide and substituted dicyandiamides.Preferably, the epoxide reactive curing agent has carboxylic acidgroups.

[0126] In one embodiment of the present invention, the epoxide reactivecrosslinking agent has carboxylic acid functionality and issubstantially crystalline. By “crystalline” is meant that theco-reactant contains at least some crystalline domains, andcorrespondingly may contain some amorphous domains. While not necessary,it is preferred that the epoxide reactive crosslinking agent have a meltviscosity less than that of the epoxy functional polymer (at the sametemperature). As used herein and in the claims, by “epoxide reactivecrosslinking agent” is meant that the epoxide reactive crosslinkingagent has at least two functional groups that are reactive with epoxidefunctionality.

[0127] Preferably, the epoxide reactive crosslinking agent is acarboxylic acid functional curing agent, which contains from 4 to 20carbon atoms. Examples of carboxylic acid functional crosslinking agentsuseful in the present invention include, but are not limited to,dodecanedioic acid, azelaic acid, adipic acid, 1,6-hexanedioic acid,succinic acid, pimelic acid, sebasic acid, maleic acid, citric acid,itaconic acid, aconitic acid and mixtures thereof.

[0128] Other suitable carboxylic acid functional curing agents includethose represented by the following general formula (VIII),

[0129] In general formula (VIII), R⁵ is the residue of a polyol, E is adivalent linking group having from 1 to 10 carbon atoms, and n is aninteger of from 2 to 10. Examples of polyols from which R⁵ of generalformula (VIII) may be derived include, but are not limited to, ethyleneglycol, di(ethylene glycol), trimethylolethane, trimethylolpropane,pentaerythritol, di-trimethylolpropane, di-pentaerythritol and mixturesthereof. Divalent linking groups from which E may be selected include,but are not limited to, methylene, ethylene, propylene, isopropylene,butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene,cyclohexylene, e.g., 1,2-cyclohexylene, substituted cyclohexylene, e.g.,4-methyl-1,2-cyclohexylene, phenylene, e.g., 1,2-phenylene, andsubstituted phenylene, e.g., 4-methyl-1,2-phenylene and 4-carboxylicacid-1,2-phenylene. The divalent linking group E is preferablyaliphatic.

[0130] The curing agent represented by general formula (VIII) istypically prepared from a polyol and a dibasic acid or cyclic anhydride.For example, trimethylol propane and hexahydro-4-methylphthalicanhydride are reacted together in a molar ratio of 1:3 respectively, toform a carboxylic acid functional curing agent. This particular curingagent can be described with reference to general formula (VIII) asfollows, R⁵ is the residue of trimethylol propane, E is the divalentlinking group 4-methyl-1,2-cyclohexylene, and n is 3. Carboxylic acidfunctional curing agents described herein with reference to generalformula VIII also are meant to include any unreacted starting materialsand/or co-products, e.g., oligomeric species, resulting from theirpreparation and contained therein.

[0131] Curable powder coating compositions comprising an epoxidefunctional polymer and an epoxide reactive curing agent can also includeone or more cure catalysts for catalyzing the reaction between thereactive functional groups of the crosslinking agent and the epoxidegroups of the polymer. Examples of cure catalysts for use with acidfunctional crosslinking agents include tertiary amines, e.g., methyldicocoamine, and tin compounds, e.g., triphenyl tin hydroxide. Whenemployed, the curing catalyst is typically present in the curable powdercoating composition in an amount of less than 5 percent by weight, e.g.,from 0.25 percent by weight to 2.0 percent by weight, based on totalweight of the composition.

[0132] Curable powder coating compositions comprising epoxide functionalpolymers and epoxide reactive curing agents typically include both in atotal amount ranging from 50 percent to 99 percent by weight, based onthe total weight of the composition, e.g., from 70 percent to 85 percentby weight, based on the total weight of the composition. The epoxidereactive curing agent can be present in the curable powder coatingcomposition in an amount corresponding to a portion of these recitedranges, i.e., 5 to 40, particularly 15 to 30, percent by weight, basedon the total weight of the composition. The equivalent ratio of epoxideequivalents in the epoxide functional polymer to the equivalents ofreactive functional groups in the curing agent is typically from 0.5:1to 2:1, e.g., from 0.8:1 to 1.5:1.

[0133] Curable powder coating compositions of the present inventioncomprising an epoxide functional polymer as reactant (1) and an epoxidereactive curing agent also comprise the crosslinking agent (2) in anamount ranging from 1 to 50 weight percent, often from 2 to 40 weightpercent and typically from 15 to 30 weight percent based on the totalweight of the powder coating composition.

[0134] The powder coating compositions of the present invention canfurther include additives as are commonly known in the art. Typicaladditives include benzoin, used to reduce entrapped air or volatiles;flow aids or flow control agents which aid in the formation of a smoothand/or glossy surface, for example, MODAFLOW® available from MonsantoChemical Co., waxes such as MICROWAX® C available from Hoechst, fillerssuch as calcium carbonate, barium sulfate and the like; pigments anddyes as colorants; UV light stabilizers such as TINUVIN® 123 or TINUVIN®900 available from CIBA Specialty Chemicals and catalysts to promote thevarious crosslinking reactions.

[0135] Such additives are typically present in the powder coatingcompositions of the present invention in an amount ranging from 1 to 20weight percent based on total weight of the powder coating composition.

[0136] The powder coating compositions of the invention are typicallyprepared by blending the functional group-containing polymer (1) and thecrosslinking agent (2) for approximately 1 minute in a Henschel bladeblender. The mixture is then extruded through an extruder, for example,a Baker-Perkins twin screw extruder, at a temperature ranging from 158°F. to 266° F. (70° C. to 130° C.). The resultant chip is usually groundand classified to an appropriate particle size, typically between 20 and200 microns, in a cyclone grinder/sifter.

[0137] The powder coating compositions of the invention can be appliedto a variety of substrates including metallic substrates, for example,aluminum and steel substrates, and non-metallic substrates, for example,thermoplastic or thermoset composite substrates. The powder coatingcompositions are typically applied by spraying, and in the case of ametal substrate, by electrostatic spraying, or by the use of a fluidizedbed. The powder coating can be applied in a single sweep or in severalpasses to provide a film having a thickness after cure of from about 1to 10 mils (25 to 250 micrometers), usually about 2 to 4 mils (50 to 100micrometers).

[0138] Generally, after application of the powder coating composition,the powder coated substrate is heated to a temperature sufficient tocure the coating, usually to a temperature ranging from 250° F. to 500°F. (121.1° C. to 260.0° C.) for 1 to 60 minutes, and typically from 300°F. to 400° F. (148.9° C. to 204.4° C.) for 15 to 30 minutes.

[0139] The powder coating composition can be applied as a primer orprimer surfacer, or as a top coat, for example, a “monocoat”. In oneembodiment, the present invention is directed to a powder coatingcomposition which is advantageously employed as a top coat in amulti-layer composite coating composition. Such a multi-componentcomposite coating composition generally comprises a base coat depositedfrom a base coat film-forming composition (which usually is pigmented)and a top coat applied over the base coat, the top coat being depositedfrom the powder coating composition of the present invention asdescribed above. In another embodiment of the present invention, themulti-component composite coating composition is a color-plus-clearsystem where the top coat is deposited from a powder coating compositionwhich is substantially pigment-free, i.e., a clear coat.

[0140] The film-forming composition from which the base coat isdeposited can be any of the compositions useful in coatingsapplications, for example, in automotive applications, wherecolor-plus-clear systems are often used. A film-forming compositionconventionally comprises a resinous binder and, typically, a pigment toserve as a colorant. Particularly useful resinous binders includeacrylic polymers, polyesters including alkyds, and polyurethanesexamples of which can include the polymers described above.

[0141] The resinous binders for the base coat can be organicsolvent-based materials, such as those described in U.S. Pat. No.4,220,679. Water-based coating compositions, such as those described inU.S. Pat. Nos. 4,403,003; 4,147,679; and 5,071,904, also can be used asthe base coat composition.

[0142] As mentioned above, the base coat compositions also containpigments of various types as colorants. Suitable metallic pigmentsinclude aluminum flake, bronze flake, copper flake and the like. Otherexamples of suitable pigments include mica, iron oxides, lead oxides,carbon black, titanium dioxide, talc, as well as a variety of colorpigments.

[0143] Optional ingredients for the base coat film-forming compositionsinclude those which are well known in the art of surface coatings andinclude surfactants, flow control agents, thixotropic agents, fillers,anti-gassing agents, organic co-solvents, catalysts and other suitableadjuvants.

[0144] The base coat film-forming compositions can be applied to thesubstrate by any of the conventional coating techniques, such asbrushing, spraying, dipping or flowing, but they most often arespray-applied. The usual spray techniques and equipment for airspraying, airless spraying and electrostatic spraying can be used.

[0145] The base coat film-forming compositions typically are applied tothe substrate such that a cured base coat having a film thicknessranging from 0.5 to 4 mils (12.5 to 100 micrometers) is formed thereon.

[0146] After forming a film of the base coat on the substrate, the basecoat can be cured or, alternatively, given a drying step in whichsolvent, i.e., organic solvent and/or water, is driven off by heating oran air drying step before application of the top coat. Suitable dryingconditions will depend on the particular base coat film-formingcomposition and on the ambient humidity with certain water-basedcompositions. In general, a drying time ranging from 1 to 15 minutes ata temperature of 75° F. to 200° F. (21° C. to 93° C.) is adequate.

[0147] The curable powder top coating composition can be applied to thebase coat by any of the methods of application described above. Asdiscussed above, the powder top coat can be applied to a cured or adried base coat before the base coat has been cured. In the latter case,the powder top coat and the base coat are cured simultaneously.

[0148] Illustrating the invention are the following examples, which arenot to be considered as limiting the invention to their details. Unlessotherwise indicated, all parts and percentages in the followingexamples, as well as throughout the specification, are by weight.

EXAMPLES

[0149] Example A describes the preparation of a polyester for use inpreparing a urethane group-containing adduct. Examples B through Edescribe the preparation of crosslinking agents in accordance with thepresent invention. The crosslinking agents were prepared by modifying anappropriate melamine-based aminoplast resin.

Example A Preparation of the Polyester (1)

[0150] Into a two-liter four-necked reaction kettle equipped with athermometer, a mechanical stirrer, nitrogen inlet, and a separator wereplaced 955.0 parts of hydrogenated Bisphenol A, 308.0 parts ofhexahydrophthalic anhydride, 1.3 parts of dibutyl tin oxide, and 1.3parts of triisodecylphosphite. The mixture was melted by heating and wasfurther heated to 230° C. while the water resulting from the reactionwas removed through the separator. The reaction was stopped when an acidvalue of 2 was achieved. The polyester had a hydroxyl number of 160 andT_(g) around 65° C.

Example B

[0151] Into a two-liter four-necked reaction kettle equipped with athermometer, a mechanical stirrer, nitrogen inlet, and means forremoving the by-product (methanol) were placed 279.3 parts of thepolyester (1) of Example A, and 340.0 parts of xylene. The mixture washeated to 110° C. and held for 2 hours under constant nitrogen sparge.It was then cooled down to 60° C. and a mixture of 95.3 parts of phenylisocyanate and 0.8 part of di-butyl tin di-laurate was added dropwisethrough an addition funnel. 100.0 parts of xylene were chargedsubsequently and the mixture was held at 90° C. until it was free of NCOfunctionality. Thereafter, 640.0 parts of Cymel® 300, 106.0 parts ofbenzoin, and 2.0 parts of p-toluenesulfonic acid were added to thekettle. The mixture was heated to 120° C. and the temperature wasmaintained while the methanol by-product was removed from the system.The reaction progress was monitored by measuring the IR spectra of themixture and was terminated when the end point was detected. It was thenconcentrated at a temperature of 100-130° C. in a vacuum of 3-50 mm Hgto remove the xylene solvent. The product thus obtained was a paleyellow solid with a softening temperature of around 40° C.

Example C

[0152] Into a two-liter four-necked reaction kettle equipped with athermometer, a mechanical stirrer, nitrogen inlet, and means forremoving the by-product (methanol) were placed 139.6 parts of thepolyester (1) of Example A, and 170.0 parts of xylene. The mixture washeated to 110° C. and held for 2 hours under constant nitrogen sparge.It was then cooled down to 60° C. and a mixture of 47.6 parts of phenylisocyanate and 0.4 part of di-butyl tin di-laurate was added dropwisethrough an addition funnel. 50.0 parts of xylene were chargedsubsequently and the mixture was held at 90° C. until it was free of NCOfunctionality. Thereafter, 320.0 parts of Cymel® 300, 38.5 parts ofisoborneol, and 1.0 part of p-toluenesulfonic acid were added to thekettle. The mixture was heated to 120° C. and the temperature wasmaintained while the methanol by-product was removed from the system.The reaction progress was monitored by measuring the IR spectra of themixture and was terminated when the end point was detected. It was thenconcentrated at a temperature of 100-130° C. in a vacuum of 3-50 mm Hgto remove the xylene solvent. The product thus obtained was a paleyellow solid with a softening temperature of around 40° C.

Example D

[0153] Into a two-liter four-necked reaction kettle equipped with athermometer, a mechanical stirrer, nitrogen inlet, and means forremoving the by-product (methanol) were placed 139.6 parts of thepolyester (1) of Example A, and 170.0 parts of xylene. The mixture washeated to 110° C. and held for 2 hours under constant nitrogen sparge.It was then cooled down to 60° C. and a mixture of 47.6 parts of phenylisocyanate and 0.4 part of di-butyl tin di-laurate was added dropwisethrough an addition funnel. 50.0 parts of xylene were chargedsubsequently and the mixture was held at 90° C. until it was free of NCOfunctionality. Thereafter, 320.0 parts of Cymel® 300, 38.5 parts ofisoborneol, 53.0 parts of benzoin, and 1.0 part of p-toluenesulfonicacid were added to the kettle. The mixture was heated to 120° C. and thetemperature was maintained while the methanol by-product was removedfrom the system. The reaction progress was monitored by measuring the IRspectra of the mixture and was terminated when the end point wasdetected. It was then concentrated at a temperature of 100-130° C. in avacuum of 3-50 mm Hg to remove the xylene solvent. The product thusobtained was a pale yellow solid with a softening temperature of around50° C.

Example E

[0154] This example describes the preparation of a crosslinking agent inaccordance with the present invention when using benzyl alcohol. Into atwo-liter four-necked reaction kettle equipped with a thermometer, amechanical stirrer, nitrogen inlet, and means for removing theby-product (methanol) are placed 139.6 parts of the polyester (1) ofExample A, and 170.0 parts of xylene. The mixture is heated to 110° C.and held for 2 hours under constant nitrogen sparge. It is then cooleddown to 60° C. and a mixture of 47.6 parts of phenyl isocyanate and 0.4part of di-butyl tin di-laurate is added dropwise through an additionfunnel. 50.0 parts of xylene are charged subsequently and the mixture isheld at 90° C. until it is free of NCO functionality. Thereafter, 320.0parts of Cymel® 300, 36.4 parts of benzyl alcohol, 53.0 parts ofbenzoin, and 1.0 part of p-toluenesulfonic acid are added to the kettle.The mixture is heated to 120° C. and the temperature is maintained whilethe methanol by-product is removed from the system. The reactionprogress is monitored by measuring the IR spectra of the mixture and isterminated when the end point is detected. It is then concentrated at atemperature of 100-130° C. in a vacuum of 3-50 mm Hg to remove thexylene solvent.

[0155] It will be appreciated by those skilled in the art that changescould be made to the embodiments described above without departing fromthe broad inventive concept thereof. It is understood, therefore, thatthis invention is not limited to the particular embodiments disclosed,but it is intended to cover modifications which are within the spiritand scope of the invention, as defined by the appended claims.

Therefore, we claim:
 1. A crosslinking agent comprising an ungelledreaction product of the following reactants: (A) at least one aminoplastresin; (B) a reactive urethane group-containing adduct which is areaction product of the following reactants: (1) at least onemono-isocyanate; and (2) at least one polyfunctional polymer havingfunctional groups reactive with the mono-isocyanate (1); and (C) atleast one compound different from (B) having active hydrogen groupsreactive with aminoplast resin (A), said compound selected from at leastone of: (i) compounds having the following structure (I):

 wherein X is aromatic; R¹, R², and R³ can be the same or different andeach independently represents H, (cyclo)alkyl having from 1 to 12 carbonatoms, aryl, alkaryl, aralkyl, or an active hydrogen-containing group,provided that at least one of R¹, R², and R³ represents an activehydrogen-containing group which is reactive with the aminoplast resin(A); (ii)compounds having the following structure (II) or (III):

 where R′ and R″ are the same or different and each independentlyrepresents an aromatic group or an alkyl group having 1 to 12 carbonatoms; and (iii) compounds different from (i) and (ii) and having amelting point of at least 80° C.,wherein said crosslinking agent isessentially free of urethane NH functionality and has a glass transitiontemperature of at least 25° C.
 2. The crosslinking agent of claim 1,wherein the aminoplast resin (A) is or is derived from at least one ofglycoluril, aminotriazine and benzoguanamine.
 3. The crosslinking agentof claim 2, wherein the aminoplast resin (A) comprises alkoxylatedaldehyde condensate of glycoluril.
 4. The crosslinking agent of claim 3,wherein the aminoplast resin (A) comprises tetramethoxymethylglycoluril.
 5. The crosslinking agent of claim 2, wherein theaminoplast resin (A) comprises (alkoxyalkyl) aminotriazine having one orless non-alkylated NH bond per triazine ring.
 6. The crosslinking agentof claim 5, wherein the aminoplast resin (A) comprises (methoxymethyl)aminotriazine.
 7. The crosslinking agent of claim 5, wherein the(alkoxyalkyl) aminotriazine has a degree of polymerization of 1.75 orless.
 8. The crosslinking agent of claim 1, wherein the polyfunctionalpolymer (2) is selected from acrylic polymers, polyester polymers,polyether polymers, and mixtures thereof.
 9. The crosslinking agent ofclaim 1, wherein the polyfunctional polymer (2) comprises functionalgroups selected from hydroxyl groups, amine groups, thiol groups, andcombinations thereof.
 10. The crosslinking agent of claim 1, wherein thepolyfunctional polymer (2) comprises a polyester polymer having hydroxylfunctional groups.
 11. The crosslinking agent of claim 10, wherein thepolyester polymer comprises a condensation reaction product of thefollowing reactants: (A) a cycloaliphatic polyol; and (B) a cyclicpolycarboxylic acid or anhydride.
 12. The crosslinking agent of claim11, wherein the cycloaliphatic polyol (A) comprises a diol selected fromhydrogenated Bisphenol A, cyclohexane dimethanol and mixtures thereof.13. The crosslinking agent of claim 11, wherein the cycloaliphaticpolyol (A) comprises hydrogenated Bisphenol A.
 14. The crosslinkingagent of claim 11, wherein the cyclic polycarboxylic acid (B) isselected from hexahydrophthalic acid, phthalic acid, isophthalic acid,terephthalic acid, anhydrides thereof, and mixtures thereof.
 15. Thecrosslinking agent of claim 14, wherein the cyclic polycarboxylic acid(B) comprises hexahydrophthalic anhydride.
 16. The crosslinking agent ofclaim 11, wherein the number average molecular weight of the polyesterpolymer ranges from 400 to
 1500. 17. The crosslinking agent of claim 10,wherein the hydroxyl value of the polyester polymer ranges from 90 to180.
 18. The crosslinking agent of claim 1, wherein the glass transitiontemperature of the polyfunctional polymer (2) is at least 15° C.
 19. Thecrosslinking agent of claim 1, wherein the glass transition temperatureof the polyfunctional polymer (2) ranges from 40° C. to 120° C.
 20. Thecrosslinking agent of claim 1, wherein the mono-isocyanate (1) isselected from cyclohexyl isocyanate, phenyl isocyanate, butyl isocyanateand mixtures thereof.
 21. The crosslinking agent of claim 20, whereinthe mono-isocyanate (1) comprises cyclohexyl isocyanate.
 22. Thecrosslinking agent of claim 1, wherein the molar ratio of thepolyfunctional polymer (2) to the mono-isocyanate (1) ranges from 1:1.8to 2.0.
 23. The crosslinking agent of claim 1, wherein the compound (C)comprises at least one compound having the structure (I).
 24. Thecrosslinking agent of claim 23, wherein at least one of R¹, R², and R³represents a group comprising an active hydrogen-containing groupselected from hydroxyl, amide, amine, carboxylic acid, carbamate, urea,thiol and mixtures thereof.
 25. The crosslinking agent of claim 24,wherein at least one of R¹, R², and R³ represents a group comprising atleast one hydroxyl group.
 26. The crosslinking agent of claim 25,wherein the compound (C) comprises benzyl alcohol.
 27. The crosslinkingagent of claim 1, wherein the compound (C) comprises at least one ofcompound (C)(ii).
 28. The crosslinking agent of claim 27, wherein thecompound (C) comprises a hydroxyl functional group-containing compoundhaving the following structure (II):

or dimer derivatives thereof, wherein R′ and R″ are the same ordifferent and each independently represents an (cyclo)alkyl group having1 to 12 carbon atoms or an aromatic group.
 29. The crosslinking agent ofclaim 28, wherein one or both of R′ and R″ represent aromatic groups.30. The crosslinking agent of claim 28, wherein the compound (C)comprises a compound selected from benzoin, hydroxycyclohexyl phenylketone and mixtures thereof.
 31. The crosslinking agent of claim 30,wherein the compound (C) comprises benzoin.
 32. The crosslinking agentof claim 30, wherein the compound (C) comprises hydroxycyclohexyl phenylketone.
 33. The crosslinking agent of claim 28, wherein one or both ofR′ and R″ are aromatic groups containing at least one heteroatomselected from furyl, pyridyl, methoxy phenyl, and dimethylaminophenylgroups.
 34. The crosslinking agent of claim 27, wherein the activehydrogen group-containing compound (C) comprises a compound having thefollowing structure (III):

wherein R′ and R″ are the same or different and each independentlyrepresents an alkyl group having 1 to 12 carbon atoms or an aromaticgroup.
 35. The crosslinking agent of claim 1, wherein the compound (C)comprises at least one of compound (C) (iii).
 36. The crosslinking agentof claim 1, wherein compound (C) comprises an aliphatic mono-functionalalcohol selected from borneol, norborneol, isoborneol,1-adamantanemethanol, 1-adamantanol, 2-methyl-2-adamantanol and5-norbornen-2-ol.
 37. The crosslinking agent of claim 1, wherein thecompound (C) is selected from benzoin, isoborneol, triphenylmethanol,N-tert-butylacrylamide, p-acetophenetidide and mixtures thereof.
 38. Acrosslinking agent comprising an ungelled reaction product of thefollowing reactants: (A) at least one aminoplast resin comprising(alkoxyalkyl) aminotriazine having one or less non-alkylated NH bond pertriazine ring; (B) at least one reactive urethane group-containingreaction product of the following reactants: (i) a monoisocyanateselected from cyclohexyl isocyanate, phenyl isocyanate, butylisocyanate, and mixtures thereof; and (ii) a hydroxyl group-containingpolyester polymer having a glass transition temperature of at least 15°C. comprising the condensation reaction product of the followingreactants: (1) a cycloaliphatic polyol; and (2) a cyclic polycarboxylicacid or anhydride; and (C) at least one compound selected from benzoin,isoborneol, benzyl alcohol, and mixtures thereof, wherein saidcrosslinking agent has a glass transition temperature of at least 10° C.and is essentially free of groups which are reactive with aminoplastresin.
 39. The crosslinking agent of claim 1, wherein the ratio of totalcombined moles of the urethane group-containing reaction product (B) andthe active hydrogen-containing compound (C) to moles of aminoplast resin(A) ranges from1:1.5to 3.2.
 40. A method for preparing a powdercrosslinking agent comprising the following steps: (1) reacting thefollowing reactants: (a) at least one mono-isocyanate (b) at least onepolyfunctional polymer having functional groups reactive with themono-isocyanate (a) in a molar ratio of reactant (b) to reactant (a)ranging from 1:1.8 to 2.0 to form a urethane group-containing reactionproduct; (2) combining the following: (A) at least one aminoplast resin;(B) the urethane group-containing reaction product formed in step (1);and (C) at least one compound different from (B) having active hydrogengroups reactive with aminoplast resin (A), said compound selected fromat least one of: (i) compounds having the following structure (I):

 wherein X is aromatic; R¹, R², and R³ can be the same or different andeach independently represents H, (cyclo)alkyl having from 1 to 12 carbonatoms, aryl, alkaryl, aralkyl, or an active hydrogen-containing group,provided that at least one of R¹, R², and R³ represents an activehydrogen-containing group which is reactive with the aminoplast resin(A); (ii) compounds having the following structure (II) or (III):

 where R′ and R″ are the same or different and each independentlyrepresents an aromatic group or an alkyl group having 1 to 12 carbonatoms; and (iii) compounds different from (i) and (ii) and having amelting point of at least 80° C.; in a ratio of total combined moles of(B) and (C) to moles of aminoplast resin (A) ranging from 1:1.5 to 3.2to form a reaction admixture; and (3) heating the reaction admixtureformed in step (2) to a temperature ranging from 95° C. to 135° C. for atime sufficient to form a crosslinking agent having a glass transitiontemperature of at least 25° C. which is essentially free of urethane NHfunctionality as determined by infrared spectroscopy.
 41. The method ofclaim 40, wherein the aminoplast resin (A) is or is derived from atleast one of glycoluril, aminotriazine and benzoguanamine.
 42. Themethod of claim 41, wherein the aminoplast resin (A) comprisesalkoxylated aldehyde condensate of glycoluril.
 43. The method of claim42, wherein the aminoplast resin (A) comprises tetramethoxymethylglycoluril.
 44. The method of claim 41, wherein the aminoplastresin (A) comprises (alkoxyalkyl) aminotriazine having one or lessnon-alkylated NH bond per triazine ring.
 45. The method of claim 44,wherein the aminoplast resin (A) comprises (methoxymethyl)aminotriazine.
 46. The method of claim 44, wherein the (alkoxyalkyl)aminotriazine has a degree of polymerization of 1.75 or less.
 47. Themethod of claim 40, wherein the polyfunctional polymer (b) is selectedfrom acrylic polymers, polyester polymers, polyether polymers, andmixtures thereof.
 48. The method of claim 40, wherein the polyfunctionalpolymer (b) comprises functional groups selected from hydroxyl groups,amine groups, thiol groups, and combinations thereof.
 49. The method ofclaim 40, wherein the polyfunctional polymer (b) comprises a polyesterpolymer having hydroxyl functional groups.
 50. The method of claim 49,wherein the polyester polymer comprises a condensation reaction productof the following reactants: (A) a cycloaliphatic polyol; and (B) acyclic polycarboxylic acid or anhydride.
 51. The method of claim 50,wherein the cycloaliphatic polyol (A) comprises a diol selected fromhydrogenated Bisphenol A, cyclohexane dimethanol and mixtures thereof.52. The method of claim 51, wherein the cycloaliphatic polyol (A)comprises hydrogenated Bisphenol A.
 53. The method of claim 50, whereinthe cyclic polycarboxylic acid (B) is selected from hexahydrophthalicacid, phthalic acid, isophthalic acid, terephthalic acid, anhydridesthereof, and mixtures thereof.
 54. The method of claim 53, wherein thecyclic polycarboxylic acid (B) comprises hexahydrophthalic anhydride.55. The method of claim 50, wherein the number average molecular weightof the polyester polymer ranges from 400 to
 1500. 56. The method ofclaim 50, wherein the hydroxyl value of the polyester polymer rangesfrom 90 to
 180. 57. The method of claim 40, wherein the glass transitiontemperature of the polyfunctional polymer (b) is at least 15° C.
 58. Themethod of claim 39, wherein the glass transition temperature of thepolyfunctional polymer (b) ranges from 40° C. to 120° C.
 59. The methodof claim 40, wherein the mono-isocyanate (a) is selected from cyclohexylisocyanate, phenyl isocyanate, butyl isocyanate and mixtures thereof.60. The method of claim 59, wherein the mono-isocyanate (a) comprisescyclohexyl isocyanate.
 61. The method of claim 40, wherein the molarratio of the polyfunctional polymer (b) to the mono-isocyanate (a)ranges from 1:1.8 to 2.0.
 62. The method of claim 40, wherein thecompound (C) comprises at least one compound having the structure (I).63. The method of claim 62, wherein at least one of R¹, R², and R³represents a group comprising an active hydrogen-containing groupselected from hydroxyl, amide, amine, carboxylic acid, carbamate, urea,thiol and mixtures thereof.
 64. The method of claim 63, wherein at leastone of R¹, R², and R³ represents a group comprising at least onehydroxyl group.
 65. The method of claim 40, wherein compound (C)comprises benzyl alcohol.
 66. The method of claim 40, wherein thecompound (C) comprises at least one of compound (C)(ii).
 67. The methodof claim 40, wherein the compound (C) comprises a hydroxyl functionalgroup-containing compound having the following structure (II):

or dimer derivatives thereof, wherein R′ and R″ are the same ordifferent and each independently represents an (cyclo)alkyl group having1 to 12 carbon atoms or an aromatic group.
 68. The method of claim 67,wherein one or both of R′ and R″ represent aromatic groups.
 69. Themethod of claim 67 wherein compound (C) comprises a compound selectedfrom benzoin, hydroxycyclohexyl phenyl ketone and mixtures thereof. 70.The method of claim 69, wherein the compound (C) comprises benzoin. 71.The method of claim 69, wherein the compound (C) compriseshydroxycyclohexyl phenyl ketone.
 72. The method of claim 67, wherein oneor both of R′ and R″ are aromatic groups containing at least oneheteroatom selected from furyl, pyridyl, methoxy phenyl, anddimethylaminophenyl groups.
 73. The method of claim 66, wherein compound(C) comprises a compound having the following structure (III):

wherein R′ and R″ are the same or different and each independentlyrepresents an alkyl group having 1 to 12 carbon atoms or an aromaticgroup.
 74. The method of claim 40, wherein the compound (C) comprises atleast one of compound (C) (iii).
 75. A method for preparing acrosslinking agent comprising the following steps: (1) reacting thefollowing reactants: (a) at least one mono-isocyanate selected fromcyclohexyl isocyanate, phenyl isocyanate, butyl isocyanate and mixturesthereof, and (b) at least one hydroxyl group-containing polyesterpolymer having a glass transition temperature of at least 15° C.comprising the condensation reaction product of the following reactants:(i) a cycloaliphatic polyol; and (ii) a cyclic polycarboxylic acid oranhydride; in a molar ratio of reactant (b) to reactant (a) ranging from1.0:1.8 to 2.0 to form a urethane group-containing reaction product; (2)combining the following: (A) at least one aminoplast resin; (B) theurethane group-containing reaction product formed in step (1); and (C)at least one compound selected from benzoin, isoborneol, benzyl alcoholand mixtures thereof, in a ratio of combined moles of (B) and (C) tomoles of aminoplast resin (A) ranging from 1.0:1.5 to 3.2 to form areaction admixture; and (3) heating the reaction admixture formed instep (2) to a temperature ranging from 95° C. to 135° C. for a timesufficient to form a crosslinking agent having a glass transitiontemperature of at least 25° C. which is essentially free of urethane NHfunctionality as determined by infrared spectroscopy.
 76. The method ofclaim 75, wherein the polyester polymer (b) comprises the reactionproduct of: (i) a cycloaliphatic polyol selected from hydrogenatedBisphenol A, cyclohexane dimethanol and mixtures thereof; and (ii) acyclic polycarboxylic acid selected from hexahydrophthalic acid,phthalic acid, isophthalic acid, terephthalic acid, anhydrides thereof,and mixtures thereof.
 77. The method of claim 75, wherein themono-isocyanate (a) comprises cyclohexyl isocyanate.
 78. The method ofclaim 68, wherein the aminoplast resin (A) comprises (alkoxyalkyl)aminotriazine having one or less non-alkylated NH bond per triazinering.
 79. A curable powder coating composition comprising a solidparticulate mixture of the following: (1) a film-forming polymer havingreactive functional groups, (2) a crosslinking agent having functionalgroups reactive with the functional groups of (1), said crosslinkingagent comprising an ungelled reaction product of the followingreactants: (A) at least one aminoplast resin; (B) a reactive urethanegroup-containing adduct which is a reaction product of the followingreactants: (1) at least one mono-isocyanate; and (2) at least onepolyfunctional polymer having functional groups reactive with themono-isocyanate (1); and (C) at least one compound different from (B)having active hydrogen groups reactive with aminoplast resin (A), saidcompound selected from at least one of: (i) compounds having thefollowing structure (I):

 wherein X is aromatic; R¹, R², and R³ can be the same or different andeach independently represents H, (cyclo)alkyl having from 1 to 12 carbonatoms, aryl, alkaryl, aralkyl, or an active hydrogen-containing group,provided that at least one of R¹, R², and R³ represents an activehydrogen-containing group which is reactive with the aminoplast resin(A), (ii) compounds having the following structure (II) or (III):

 where R′ and R″ are the same or different and each independentlyrepresents an aromatic group or an alkyl group having 1 to 12 carbonatoms; and (iii) compounds different from (i) and (ii) and having amelting point of at least 80° C.; wherein said crosslinking agent isessentially free of urethane NH functionality and has a glass transitiontemperature of at least 25° C.
 80. The curable powder coatingcomposition of claim 79, wherein the polymer (1) is selected fromacrylic, polyester, polyepoxide, polyurethane and polyether polymers andmixtures thereof.
 81. The curable powder coating composition of claim79, wherein the polymer (1) comprises hydroxyl and/or carbamatefunctional groups.
 82. The curable powder coating composition of claim79, wherein the polymer (1) comprises hydroxyl and/or epoxy functionalgroups.
 83. The curable powder coating composition of claim 79, whereinthe polymer (1) is present in the composition in an amount ranging from5 to 90 percent by weight based on total weight of the composition. 84.The curable powder coating composition of claim 79, wherein thepolyfunctional polymer (2) comprises a polyester polyol comprising thecondensation reaction product of the following reactants: (A) acycloaliphatic polyol; and (B) a cyclic polycarboxylic acid oranhydride.
 85. The curable powder coating composition of claim 84,wherein the cycloaliphatic polyol (A) is selected from hydrogenatedBisphenol A, cyclohexane dimethanol, cyclohexane diol and mixturesthereof.
 86. The curable powder coating composition of claim 84, whereinthe cyclic polycarboxylic acid (B) is selected from hexahydrophthalicacid, isophthalic acid, phthalic acid, terephthalic acid, anhydridesthereof, and mixtures thereof.
 87. The curable powder coatingcomposition of claim 79, wherein the mono-isocyanate (1) is selectedfrom cyclohexyl isocyanate, phenyl isocyanate, butyl isocyanate andmixtures thereof.
 88. The curable powder coating composition of claim79, wherein the molar ratio of the polyfunctional polymer (2) to themono-isocyanate (1) ranges from 1:1.8 to 2.0.
 89. The curable powdercoating composition of claim 79, wherein the aminoplast resin (A) is oris derived from at least one of glycoluril, aminotriazine andbenzoguanamine.
 90. The curable powder coating composition of claim 89,wherein the aminoplast resin (A) comprises an alkoxylated aldehydecondensate of glycoluril.
 91. The curable powder coating composition ofclaim 90, wherein the aminoplast resin (A) comprises tetramethoxymethylglycoluril.
 92. The curable powder coating composition of claim89, wherein the aminoplast resin (A) comprises (alkoxyalkyl)aminotriazine having one or less non-alkylated NH bond per triazinering.
 93. The curable powder coating composition of claim 92, whereinthe aminoplast resin (A) comprises (methoxymethyl) aminotriazine. 94.The curable powder coating composition of claim 92, wherein the(alkoxyalkyl) aminotriazine has a degree of polymerization of 1.75 orless.
 95. The curable powder coating composition of claim 79, whereinthe polyfunctional polymer (2) is selected from acrylic polymers,polyester polymers, polyether polymers, and mixtures thereof.
 96. Thecurable powder coating composition of claim 79, wherein thepolyfunctional polymer (2) comprises functional groups selected fromhydroxyl groups, amine groups, thiol groups, and combinations thereof.97. The curable powder coating composition of claim 79, wherein thepolyfunctional polymer (2) comprises a polyester polymer having hydroxylfunctional groups.
 98. The curable powder coating composition of claim97, wherein the polyester polymer comprises a condensation reactionproduct of the following reactants: (A) a cycloaliphatic polyol; and (B)a cyclic polycarboxylic acid or anhydride.
 99. The curable powdercoating composition of claim 98, wherein the cycloaliphatic polyol (A)comprises a diol selected from hydrogenated Bisphenol A, cyclohexanedimethanol and mixtures thereof.
 100. The curable powder coatingcomposition of claim 99, wherein the cycloaliphatic polyol (A) compriseshydrogenated Bisphenol A.
 101. The curable powder coating composition ofclaim 98, wherein the cyclic polycarboxylic acid (B) is selected fromhexahydrophthalic acid, phthalic acid, isophthalic acid, terephthalicacid, anhydrides thereof, and mixtures thereof.
 102. The curable powdercoating composition of claim 101, wherein the cyclic polycarboxylic acid(B) comprises hexahydrophthalic anhydride.
 103. The curable powdercoating composition of claim 98, wherein the number average molecularweight of the polyester polymer ranges from 400 to
 1500. 104. Thecurable powder coating composition of claim 98, wherein the hydroxylvalue of the polyester polymer ranges from 90 to
 180. 105. The curablepowder coating composition of claim 79 wherein the glass transitiontemperature of the polyfunctional polymer (2) is at least 15° C. 106.The curable powder coating composition of claim 79, wherein the glasstransition temperature of the polyfunctional polymer (2) ranges from 40°C. to 120° C.
 107. The curable powder coating composition of claim 79,wherein the mono-isocyanate (1) is selected from cyclohexyl isocyanate,phenyl isocyanate, butyl isocyanate and mixtures thereof.
 108. Thecurable powder coating composition of claim 107, wherein themono-isocyanate (1) comprises cyclohexyl isocyanate.
 109. The curablepowder coating composition of claim 79, wherein the molar ratio of thepolyfunctional polymer (2) to the mono-isocyanate (1) ranges from 1:1.8to 2.0.
 110. The curable powder coating composition of claim 79, whereinthe compound (C) comprises at least one of compound (C)(i).
 111. Thecurable powder coating composition of claim 110, wherein at least one ofR¹, R², and R³ represents a group comprising an activehydrogen-containing group selected from hydroxyl, amide, amine,carboxylic acid, carbamate, urea, thiol, and mixtures thereof.
 112. Thecurable powder coating composition of claim 110, wherein at least one ofR¹, R², and R³ represents a group comprising at least one hydroxylgroup.
 113. The curable powder coating composition of claim 110, whereincompound (C) comprises benzyl alcohol.
 114. The curable powder coatingcomposition of claim 79, wherein the compound (C) comprises at least oneof compound (C)(ii).
 115. The curable powder coating composition ofclaim 114, wherein the compound (C) comprises a hydroxyl functionalgroup-containing compound having the following structure (II):

or dimer derivatives thereof, wherein R′ and R″ are the same ordifferent and each independently represents an (cyclo)alkyl group having1 to 12 carbon atoms or an aromatic group.
 116. The curable powdercoating composition of claim 115, wherein one or both of R′ and R″represent aromatic groups.
 117. The curable powder coating compositionof claim 115, wherein compound (C) comprises a compound selected frombenzoin, hydroxycyclohexyl phenyl ketone and mixtures thereof.
 118. Thecurable powder coating composition of claim 117, wherein the compound(C) comprises benzoin.
 119. The curable powder coating composition ofclaim 117, wherein the compound (C) comprises hydroxycyclohexyl phenylketone.
 120. The curable powder coating composition of claim 115,wherein one or both of R′ and R″ are aromatic groups containing at leastone heteroatom selected from furyl, pyridyl, methoxy phenyl, anddimethylaminophenyl groups.
 121. The curable powder coating compositionof claim 114, wherein compound (C) comprises a compound having thefollowing structure (III):

wherein R′ and R″ are the same or different and each independentlyrepresents an alkyl group having 1 to 12 carbon atoms or an aromaticgroup.
 122. The curable powder coating composition of claim 79, whereinthe compound (C) comprises a compound selected from isoborneol,triphenylmethanol, N-tert-butylacrylamide, p-acetophenetidide andmixtures thereof.
 123. The curable powder coating composition of claim79, wherein the crosslinking agent (2) is present in an amount rangingfrom 5 to 95 weight percent based on total weight of the composition.124. A multi-layer composite coating comprising a pigmented base coatdeposited from a base coat film-forming composition and a transparenttop coat over at least a portion of the base coat, said top coatdeposited from the curable powder coating composition of claim
 79. 125.A substrate coated with the curable powder coating composition of claim79.
 126. A substrate coated with the multi-layer composite coating ofclaim 124.